Comparative removal of antibiotic resistance genes during chlorination, ozonation, and UV treatment

Efficient treatment methods for the removal of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) from drinking water are needed to reduce health risks. However, there is a lack of empirical data on ARB and ARG removal during conventional water disinfection processes. In this study, the removal of ARB and ARGs by three disinfection processes (chlorination, ozonation, and UV treatment) was investigated on a laboratory scale using Escherichia coli and Enterococcus faecium carrying ARGs. Bacterial inactivation was determined by plate count methods, and ARG damage was quantified using real-time PCR. Only for ozone treatment, similar inactivation rates for bacterial cells and ARGs were observed when 1 mg*L^-1 of ozone, with a contact time of 5 min, was used, which resulted in a 5.0 log reduction of bacterial cells and a 4.3-4.6 log reduction of ARGs. For chlorine and UV, inactivation of bacterial cells was observed at lower doses than those needed for the decrease of ARG copy numbers. The use of 0.5 mg*L^-1 free chlorine (30 min contact time) led to a 3.8-5.6 log reduction of the bacterial numbers and to a 0.8-2.8 log reduction of ARGs. Ultraviolet light irradiation with 600 J*m^-2 resulted in a 4.8-5.5 log reduction of bacterial cells, but in a negligible reduction (0-1.0 log) of ARGs. Although UV and chlorine treatments were effective in the inactivation of bacterial cells, incomplete degradation of ARGs was observed. Therefore, plasmid-borne ARGs can potentially be transferred to other bacteria even after the disinfection process. Our results provide important insights into the fate of ARGs during drinking water disinfection processes.